Lithographic apparatus, and device manufacturing method

ABSTRACT

A lithographic apparatus configured to project a patterned beam of radiation onto a target portion of a substrate is disclosed. The apparatus includes a first radiation dose detector and a second radiation dose detector, each detector comprising a secondary electron emission surface configured to receive a radiation flux and to emit secondary electrons due to the receipt of the radiation flux, the first radiation dose detector located upstream with respect to the second radiation dose detector viewed with respect to a direction of radiation transmission, and a meter, connected to each detector, to detect a current or voltage resulting from the secondary electron emission from the respective electron emission surface.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 11/544,930 filed on Oct. 10, 2006, (that issued as U.S. Pat.No. 7,629,594 on Dec. 8, 2009), the contents of which are incorporatedherein by reference in their entirety

BACKGROUND

1. Field of the Invention

The present invention relates to an apparatus, and a method formanufacturing a device.

2. Related Art

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (1Cs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

Molecular contamination by, for instance, carbon on optical componentsin a lithographic projection apparatus (e.g. grazing incidence andmulti-layer mirrors in an EUV lithographic projection apparatus) may bea problem. For example, contamination of reflective elements in an EUVlithographic projection apparatus may be caused by the presence ofhydrocarbons and electrons that are generated by EUV illumination. Afurther problem may be how to monitor the dose of radiation from asource and the amount of contamination that gathers on an opticalcomponent.

SUMMARY

A lithographic projection apparatus may be provided wherein an object(e.g., an optical element) situated in a pulsed beam of radiation has anelectrode in its vicinity and a voltage source connected either to theelectrode or to the object. The source may provide, for example, anegative voltage pulse to the object relative to the electrode. The beamof radiation and the voltage pulse from the voltage source are providedin phase or out of phase. In this way, the object is shielded againstsecondary electrons generated by the radiation beam. A measuring deviceconfigured to measure the current generated by secondary electrons inthe electrode may be provided. The dose of radiation from a source andthe amount of contamination that gathers on the body may be monitored bymeasuring the electron flux from the body. An amount of secondaryelectrons collected may be a measure for the dose of radiation and theamount of contamination. The measurement may be easily determined usinga current measuring device connected to the electrode or the object.

One or more embodiments of the present invention include an improvedlithographic apparatus wherein contamination can be detected accurately,in a relatively simple manner.

According to an embodiment, there is provided a lithographic apparatusconfigured to project a patterned beam of radiation onto a targetportion of a substrate, the apparatus comprising:

a first radiation dose detector and a second radiation dose detector,each detector comprising a secondary electron emission surfaceconfigured to receive a radiation flux and to emit secondary electronsdue to the receipt of the radiation flux, the first radiation dosedetector located upstream with respect to the second radiation dosedetector viewed with respect to a direction of radiation transmission;and

a meter, connected to each detector, to detect a current or voltageresulting from the secondary electron emission from the respectiveelectron emission surface.

According to an embodiment, there is provided a lithographic apparatusconfigured to project a patterned beam of radiation onto a targetportion of a substrate, the apparatus comprising:

a radiation dose detector substantially insensitive to a contaminantthat is likely to contaminate the detector during operation of theapparatus, the detector comprising a secondary electron emission surfaceconfigured to receive a radiation flux, and which may also receive thecontaminant, and to emit secondary electrons due to the receipt of theradiation flux, the surface substantially made of the contaminant or amaterial with a secondary electron emission similar to that of thecontaminant; and

a meter, connected to the surface, to detect a current or voltageresulting from the secondary electron emission.

According to an embodiment, there is provided a lithographic apparatusconfigured to project a patterned beam of radiation onto a targetportion of a substrate, the apparatus comprising:

a radiation dose detector sensitive to a contaminant that is likely tocontaminate the detector during operation of the apparatus, the detectorcomprising a secondary electron emission surface configured to receive aradiation flux, and which can also receive the contaminant, and to emitsecondary electrons due to the receipt of the radiation flux, thedetector positioned to receive a contaminant emanating from thesubstrate during operation; and

a meter, connected to the detector surface, to detect a current orvoltage resulting from the secondary electron emission.

According to an embodiment, there is provided a lithographic apparatusconfigured to project a patterned beam of radiation onto a targetportion of a substrate, the apparatus comprising:

a radiation dose detector sensitive to a contaminant that is likely tocontaminate the detector during operation of the apparatus, the detectorcomprising a secondary electron emission surface configured to receive aradiation flux, and which can also receive the contaminant, and to emitsecondary electrons due to the receipt of the radiation flux, thedetector positioned to receive a contaminant emanating from thesubstrate during operation; and

a meter, connected to the detector surface, to detect a current orvoltage resulting from the secondary electron emission.

According to an embodiment, there is provided a lithographic apparatusconfigured to project a patterned beam of radiation onto a targetportion of a substrate, the apparatus comprising:

an etch detector comprising a detector body having a secondary electronemission surface, the surface configured to receive a radiation flux andto emit secondary electrons due to the receipt of the radiation flux,wherein the composition of the detector body varies in a directionperpendicular from the surface;

a meter, connected to the detector body, to detect a current or voltageresulting from the secondary electron emission; and a data processorconfigured to detect etching of the detector from the measurement by themeter.

According to an embodiment, there is provided a lithographic method,comprising:

projecting a patterned beam of radiation onto a target portion of asubstrate; and

detecting a radiation dose utilizing a radiation dose detectorsubstantially insensitive to a contaminant that is likely to contaminatethe detector during operation of the apparatus, the detector comprisinga secondary electron emission surface substantially made of thecontaminant or a material with a secondary electron emission similar tothat of the contaminant, wherein radiation received by the surfaceresults in emission of secondary electrons from the surface and acurrent or voltage resulting from the secondary electron emission isdetected, the current or voltage being independent of the presence ofthe contaminant on the surface.

According to an embodiment, there is provided a method to clean (forexample by removing a layer of contaminants) a surface of an opticalcomponent, the method comprising:

subjecting the surface to a cleaning treatment; and monitoring thesurface by irradiating the surface such that secondary electrons areemitted by the surface and detecting a resulting secondary emissionvoltage or current.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts an embodiment of a lithographic apparatus;

FIG. 2 schematically depicts an embodiment of a dose and/orcontamination detection system;

FIG. 3 schematically depicts another embodiment of an energy and/orcontamination detection system;

FIG. 4 schematically depicts a further embodiment of an energy and/orcontamination detection system;

FIG. 5 schematically depicts results of an experiment;

FIG. 6 schematically depicts another embodiment of an energy and/orcontamination detection system;

FIG. 7 schematically depicts a further embodiment of an energy and/orcontamination detection system;

FIG. 8 schematically depicts an embodiment of an energy and/orcontamination detection system; and

FIG. 9 schematically depicts a detail of the embodiment of FIG. 8.

DETAILED DESCRIPTION

FIG. 1 schematically. depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

-   -   an illumination system (illuminator) IL configured to condition        a radiation beam B (e.g. UV radiation or other types of        radiation).    -   a support structure (e.g. a mask table) MT constructed to        support a patterning device (e.g. a mask) MA and connected to a        first positioner PM configured to accurately position the        patterning device in accordance with certain parameters;    -   a substrate table (e.g. a wafer table) WT constructed to hold a        substrate (e.g. a resist-coated wafer) W and connected to a        second positioner PW configured to accurately position the        substrate in accordance with certain parameters; and    -   a projection system (e.g. a refractive projection lens system)        PS configured to project a pattern imparted to the radiation        beam B by patterning device MA onto a target portion C (e.g.        comprising one or more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a reflective type (e.g. employinga reflective mask). Alternatively, the apparatus may be of atransmissive type (e.g. employing a transmissive mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more support structures). In such“multiple stage” machines the additional tables and/or supportstructures may be used in parallel, or preparatory steps may be carriedout on one or more tables and/or support structures while one or moreother tables and/or support structures are being used for exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery systemcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem if required, may be referred to as a radiation system.

In an embodiment, the radiation source is a plasma EUV source, forexample a tin (Sn) plasma EUV source. For example, in such a radiationsource, atoms can be heated (e.g., electrically) using a low powerlaser, or in a different manner. The EUV radiation source may also be adifferent radiation source, for example a Li or Xe ‘fueled’ plasmaradiation source. Also, during use, small amounts of plasma may escapefrom the source SO, towards a collector K and the illuminator IL. Thecollector K collects radiation from the radiation source SO and isarranged to transmit the collected radiation to the illumination systemIL. Particularly, the collector K may be arranged to focus incomingradiation, received from the radiation source, onto a small focus areaor point.

The illuminator IL may comprise an adjuster to adjust the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as a-outer anda-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator and acondenser. The illuminator may be used to condition the radiation beam,to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT and ispatterned by the patterning device. Being reflected on the patterningdevice MA, the radiation beam B passes through the projection system PS,which focuses the beam onto a target portion C of the substrate W. Withthe aid of the second positioner PW and position sensor IF2 (e.g. aninterferometric device, linear encoder or capacitive sensor), thesubstrate table WT can be moved accurately, e.g., so as to positiondifferent target portions C in the path of the radiation beam B.Similarly, the first positioner PM and another position sensor IF1 canbe used to accurately position the patterning device MA with respect tothe path of the radiation beam B, e.g. after mechanical retrieval from amask library, or during a scan. In general, movement of the supportstructure MT may be realized with the aid of a long-stroke module(coarse positioning) and a short-stroke module (fine positioning), whichform part of the first positioner PM. Similarly, movement of thesubstrate table WT may be realized using a long-stroke module and ashort-stroke module, which form part of the second positioner PW. In thecase of a stepper (as opposed to a scanner) the support structure MT maybe connected to a short-stroke actuator only, or may be fixed.Patterning device MA and substrate W may be aligned using patterningdevice alignment marks M1, M2 and substrate alignment marks P1, P2.Although the substrate alignment marks as illustrated occupy dedicatedtarget portions, they may be located in spaces between target portions(these are known as scribe-lane alignment marks). Similarly, insituations in which more than one die is provided on the patterningdevice MA, the patterning device alignment marks may be located betweenthe dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at once (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the supportstructure MT may be determined by the (de-)magnification and imagereversal characteristics of the projection system PS. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratetable WT is moved or scanned while a pattern imparted to the radiationbeam is projected onto a target portion C. In this mode, generally apulsed radiation source is employed and the programmable patterningdevice is updated as required after each movement of the substrate tableWT or in between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIG. 2 schematically depicts part of an embodiment of a lithographicapparatus, for example an apparatus as described above. In the FIG. 2embodiment, the apparatus comprises a first radiation dose detector 10Aand a second radiation dose detector 10B (although more detectors may beprovided). Each of the detectors 10 comprises a secondary electronemission surface 11, the surface 11 being configured to receive aradiation flux and/or contamination, and to emit secondary electrons(depicted by arrows e) due to receipt of radiation flux. Particularly,the emission surface 11 is configured to receive a radiation flux andcontamination at the same time, during use. The radiation flux (orradiation) is schematically indicated by broken line R, and may be theabove-described radiation beam or a part thereof (emanating from theradiation source SO). As a non limiting example, the radiation beam maybe a beam of EUV radiation.

In an embodiment, the surface part of each detector surface 11 thatintersects or receives the radiation flux R is relatively small, so thatthe detectors 10 do not substantially hinder overall transmission of theradiation. For example, each detector 10 may be a small object, pin,wire element or other suitable detector structure, which may be locatedin the path of the radiation flux R. In a simple embodiment, eachdetector 10 may be in a fixed position, for example, firmly mounted to aframe of the apparatus. Alternatively, the detectors 10 may be providedwith a mechanism to move the detectors 10 into and out of the path ofthe radiation flux R, to detect radiation in desired measuring periods.Each detector surface 11 may have any of a various number of shapes,such as a plane or curved shape.

The detector surface 11 may comprise any of various materials suitableto provide the secondary electron emission, as will be appreciated bythe skilled person. In a an embodiment, the detectors 10 may be locatedin an environment where it is known that one or more types ofcontaminants might be present therein. In that case, it is advantageousif the detector surfaces 11 also comprise or consist of the samecontaminant(s) before use, so that electron emission will not besignificantly influenced by further detector contamination with thecontaminant(s) during use. For example, in the case that carbon (C)contamination might be present during use near the detector surfaces 11,these surfaces 11, in an embodiment, may already comprise or consist ofcarbon material. In the case that one or more metal contaminants (forexample, tin) might be present in the detector environment, the detectorsurfaces 11, in an embodiment, may already comprise or consist of thesame metal contaminant(for example, tin). As a further example, in anembodiment, a detector structure 10 might comprise or consist of anexpected contaminant material, or be coated with such material. In anembodiment, the surfaces 11 of the detectors 10 are made of the samematerial(s), although this is not essential.

In the present embodiment, two current (or voltage) meters 13 areprovided, and are directly or indirectly connected to the detectorsurfaces 11 in a suitable manner, to detect currents (or voltage, e.g.,voltage difference) resulting from the secondary electron emission fromeach of these surfaces 11. The meters 13 (or ‘secondary electronmeters’) may be coupled to the detectors 10 in a way, such thatsecondary electron emission from the respective detector surface 11results in a current (or voltage) that can be detected by the meter 13.For example, each meter 13 may simply be grounded, so that ejection ofelectrons from the emission surface 11 may be compensated via the meter13 from ground. Each meter 13 as such may be configured in various ways,and may be designed to detect an electrical signal relating to thesecondary electron emission (for example, current) directly orindirectly. As an example, the meter 13 may comprise a resistor, whereinthe secondary electron emission can simply be measured from the currentflowing through the resistor. Also, the meter may comprise or be avoltage meter, for example to measure a voltage across theabove-mentioned resistor, the voltage being indicative of the secondaryelectron emission.

In an embodiment, the first radiation dose detector 10A is locatedupstream with respect to the second radiation dose detector 10B, viewedwith respect to a direction of radiation transmission. Advantageously,an optical component 18 of the apparatus is located between the firstand second radiation dose detectors 10A, 10B, viewed in a radiationtransmission direction, such that the radiation can first be detected bythe first radiation dose detector 10A, then reaches the opticalcomponent 18 and after that can be detected by the second radiation dosedetector 10B. In other words, the first detector 10A is located upstreamwith respect to the optical component 18, and the second detector 10B islocated downstream with respect to the optical component 18. In anembodiment, the detectors 10A, 10B are located near the respectiveoptical component 18. Each detector 10A, 10B may be arranged in arespective line of sight of part of the optical component 18, forexample a part of the optical component 18 that is to receive, reflect,absorb and/or transmit at least part of the radiation flux R.

The optical component 18 may include any of various component types. Inthe present application, the term “optical component” may include one ormore selected from the following components 18: a mirror, a lens, acollector, a filter, a mask, an illumination system, or energy sensitivedevice or sensor (e.g. to detect radiation to provide energy and/orintensity measurements). The optical component (and particularly one ormore optical surfaces thereof) may be configured to direct, shape, orcontrol the radiation, or to detect the radiation. For example, theoptical surface of the respective optical element/component may be aradiation reflecting surface (in the case of a mirror element), atransmissive surface, or a surface having radiation absorbing parts. Asan example, indicated in FIG. 1, there may be provided a pair ofradiation dose detectors 10A, 10B to detect radiation upstream anddownstream with respect to a collector K. Also, there may be provided apair of radiation dose detectors 10B, 10C to detect radiation upstreamand downstream with respect to an illumination system IL. Similarly, apair of radiation dose detectors 10C, 10D may be implemented to detectradiation upstream and downstream with respect to a patterning deviceMA. Moreover, a pair of radiation dose detectors 10D, 10E may beprovided to detect radiation upstream and downstream of a projectionsystem PS. It will be clear that a single optical component 18, or agroup of optical components, and at least one upstream and at least onedownstream radiation dose detector 10 may be associated with each other.

There may be provided a data processor 15 which is configured to comparemeasurement results of the meters 13 to detect a change in radiationreceived by the upstream and downstream radiation dose detectors 10A,10B. The data processor 15 may be configured in any of various ways andmay be connected to the meters 13 by suitable wiring, communicationlines, and/or wirelessly (see FIG. 3), to receive measurement resultsfrom the meters 13. As an example, the data processor may includesuitable hardware, software, a computer, microcontroller,microelectronics, and/or one or more data processing modules as will beappreciated by the skilled person. For example, the data processor 15may comprise a memory to store the measurement results, for examplecontinuously or at certain time intervals. During operation, newmeasurement results may be compared with previous, stored, results,particularly to track and/or detect changes in the secondary electronemission measurements of the two detectors 10A, 10B, which changes mightindicate malfunctioning or contamination of the intermediate opticalcomponent 18 or intermediate group of optical components.

In an embodiment, the data processor may compare measurement results ofthe meters 13 during a certain operational period of the apparatus. Inthis way, radiation losses due to degeneration of the optical component18 may be detected in a simple but very accurate manner, usinginexpensive means. In an embodiment, in case a certain threshold ofdegeneration of the optical component 18 is determined, a suitablemessage or alarm may be generated, for example to alert an operator thatthe optical component should be replaced or cleaned.

The data processing of the measurement results may include variousoperations. For example, initial meter measurement results relating tosecondary electron emission from the detector surfaces 11 when theoptical component 18 is not degenerated (for example contaminated) maybe stored and used subsequently to determine any subsequent changes inthe meter measurement results that may indicate optical componentdegeneration. As an example, in the case that the measurement relatingto electron emission from the second detector 10B shows a much largerdrop, with respect to the respective initial value, than the measurementrelating to electron emission from the first detector 10A, this mayindicate radiation transmission loss due to degradation of the opticalcomponent 18. For example, during operation, the data processor 15 maysimply determine the ratio of the current measurements of the two meters13, and compare that ratio with an initial measurement ratio todetermine radiation transmission loss between the detectors 10A, 10B.For example, in an embodiment, a change in the ratio of radiationreceived by the radiation dose detectors may be determined (for exampleby the data processor 15).

For example, during operation, a radiation flux R is directed to theoptical component 18 (for example intermittently, in case of radiationpulses) and is transmitted to the downstream detector 10B, resulting insecondary electron emission from the surface 11 of that detector 10Bwhich is measured by the respective meter 13. Contamination of theoptical component 18 may lead to a loss of radiation transmission to thedownstream detector 10B, resulting in a decrease of the secondaryelectron emission. A resulting decrease of the meter measurement result,relating to the secondary electron emission from the second detector10B, might be used as evidence of the contamination of the opticalcomponent 18, but is not accurate due to the possibility that thedownstream detector 10B itself might be contaminated also andadditionally because the intensity of the radiation flux R may vary intime. In a worst case, only the downstream detector 10B is contaminated,leading to loss of secondary electron emission from the respectivesurface 11, and not the optical component 18, so that a false alarmmight be generated. Thus, a much more precise monitoring is provided, bytaking into account secondary electron emission of the upstream detector10A as well (the upstream detector 10A also receives the radiation fluxR during use), as is explained above. Moreover; in the presentembodiment, transmission loss concerning an optical component 18 may bedetected without the respective optical component 18 being part of theradiation flux detector.

FIG. 3 depicts another embodiment comprising an upstream and downstreamdetector. In this case, the optical component 18 is the upstreamdetector, the surface which can emit secondary electrons when thesurface receives the radiation flux. The optical component 18 is coupledto a meter 13 to detect the respective current (or voltage) resultingfrom the secondary electron emission. Also, a downstream secondaryelectron emission radiation flux detector 10 is provided (as in the FIG.2 embodiment). The operation of the FIG. 3 embodiment may besubstantially the same as that of the FIG. 2 embodiment, with thedifference being that the optical component 18 is now used as one of thetwo radiation dose detectors, instead of the first detector 10A shown inFIG. 2. Herein, for example, the measurement results of secondaryelectron emission from the separate detector 10 may be used to verifysuch measurement results of secondary electron emission from the opticalcomponent 18, for example to detect contamination of the opticalcomponent. Also, in this case, the separate detector 10 and opticalcomponent 18 may have secondary electron emission surfaces made of thesame material(s), to provide a similar sensitivity to contaminants.

During an experiment regarding the above-described secondary electronemission, in the situation of irradiation of a mirror or otherconductive surface with EUV, it was found out that a) the secondaryemission scales linearly with input EUV power, and b) only the top layerof the irradiated body has significant influence on the secondaryelectron emission and so the secondary electron emission virtually doesnot depend on material extending below the top layer.

The above-described detector 10 may be used as, for example, an energysensor. In that case, the detector surface material may be matched withexpected contamination at a certain position in the apparatus. Forexample, near a radiation source which may produce Sn contamination, thedetector surface may be made of Sn or SnOx. Where, for example, there isa carbonizing environment in the optical train of the apparatus, thedetector 10 may have a surface layer of, or contain, a natural carbon.For example, when the surface of one or more mirrors may be used as asensor, then the surface may be carbonized.

Thus, in an embodiment, there may be provided a contaminant-insensitiveradiation dose detector 10, 18 being substantially insensitive to acontaminant that is likely to contaminate the detector during operationof the apparatus, the contaminant-insensitive detector 10, 18 beingprovided by a secondary electron emission surface, configured to receivea radiation flux and which may also receive the contaminant, andconfigured to emit secondary electrons due to the receipt of theradiation flux, wherein the detector surface is substantially made ofthe contaminant, or a material with a secondary electron emissionsimilar to that of the contaminant. As an example, the surface may bethe surface of a radiation reflector 18. The contaminant may be, forexample, carbon, tin, tin oxide, zinc, zinc oxide, manganese, manganeseoxide, tungsten, and/or tungsten oxide.

Thus, during use, a lithographic method may comprise projecting apatterned beam of radiation onto a target portion of a substrate anddetecting a radiation dose utilizing a contaminant-insensitive radiationdose detector 10. In that case, radiation may be received by thedetector surface 11, resulting in emission of secondary electrons fromthat surface, wherein a current or voltage resulting from the secondaryelectron emission is detected, the current or voltage being independentof the presence of the contaminant on the detector surface.

As will be explained below, the detector 10 may be used to monitor acleaning process.

Thus, a very simple sensor principle is provided, which is not sensitiveto deterioration and thus may be used in monitoring of energy,contamination and/or cleaning. The sensor is flexible and cheaper than,for example, a diode sensor system.

FIG. 4 shows another embodiment, which may be combined with one or moreof the above-described embodiments, if desired. In the FIG. 4embodiment, the lithographic apparatus comprises a contaminant sensitiveradiation dose detector 10, being sensitive to at least one contaminantthat is likely to contaminate the detector during operation of theapparatus. The detector 10 comprises a secondary electron emissionsurface 11 configured to receive a radiation flux R and which may alsoreceive the contaminant, and configured to emit secondary electrons dueto the receipt of the radiation flux. Further, a meter 13 is connectedto the detector surface 11 to detect a current or voltage resulting fromthe secondary electron emission. Further, the apparatus is provided witha cleaning system 30 to remove contamination from the detector surfaceand optionally from, for example, a nearby optical component (notdepicted in FIG. 4).

The cleaning system 30 provides an advantage that contamination may beremoved from the detector surface 11. This cleaning system isparticularly advantageous when the detector 10 is used to measurecontamination, which means, that after it has become contaminated, itshould be cleaned.

For example, the cleaning system 30 may comprise an inlet 31 forhydrogen gas and a device 32 to generate hydrogen radicals (such as ahot filament, or a RF field).

According to an embodiment, the cleaning system may be configured tocarry out a method comprising providing H₂ containing gas in at leastpart of the apparatus, producing hydrogen radicals from the H₂containing gas, and having the detector surface 11 come into contactwith at least part of the hydrogen radicals to remove at least part of adeposition from that surface. As an example, the deposition may compriseone or more elements selected from B, C, Si, Ge or Sn. At least part ofthe hydrogen radicals may be generated from the H₂ containing gas by afilament, a plasma, radiation, or a catalyst configured to convert H₂into hydrogen radicals. The H₂ containing gas may further comprise ahalogen gas.

In an embodiment, the detector 10 may be used as a cleaning monitor,wherein cleaning of the detector surface 11 provides an indication ofprogress of the cleaning of the nearby optical component. As partthereof, the above-described cleaning method may be carried out on theoptical component and/or on the detector 10.

In an embodiment, the cleaning system 30 may be designed to clean partof a nearby optical component, e.g. via hydrogen (H) cleaning to removecarbon (C) from the optical component. In that case, the detectorsurface 11 may comprise a layer of contaminant on top of a layer of adifferent material than the expected contaminant. During the cleaningprocess, a step in the secondary emission signal of the detector 10 isexpected after a deposited contaminant (e.g. natural carbon or Sn) hasbeen removed from the detector surface and the pure surface material isexposed.

The step may also provide an indication of the amount of cleaning of anearby optical component. For example, the detector 10 may be arranged,with respect to the optical component, such that the detector surface 11and the optical component may receive substantially the same level ofcontamination during operation. Also, the detector 10, optical componentand cleaning system 30 may be arranged such that the cleaning processleads to substantially equal cleaning rates of the detector surface 10and optical component. Then, a detected completed detector cleaning mayprovide an indication that the optical component 10 has reached adesired cleaned state as well.

A system comprising a detector 10 and cleaning system 30 may be locatedin any of various parts of, or associated with, the lithographicapparatus, for example near the source SO, collector K, illuminationoptics, and/or projection optics. The apparatus may be provided with oneor more cleaning devices to clean one or more optical components. Forexample, the cleaning device may be movable between various positions inthe apparatus to clean various parts.

In an embodiment, the detector surface 11 substantially comprisesRuthenium (Ru). Accurate measurements may be obtained in this manner.For example, FIG. 5 shows the results of an experiment or simulationwherein a multi-layer mirror was used as the detector 10. The mirror wasa Ru-capped multi-layer mirror. A thin layer of carbon (C) was grownonto this mirror (as will occur in carbon contamination), whilemeasuring the SE (secondary electron emission) signal of themirror/detector and EUV reflectivity of the mirror surface. In FIG. 5,it can be seen that while making a transition from a desired clean Rutop surface to a ‘contaminated’ C top surface, the SE signal changessignificantly and promptly, by 50% on the first 1% EUV reflectively loss(equal to or approximately 2 nm C). Thus, if linearity is assumed thenthe accuracy of the detector is in the order of 0.25% SE signalaccuracy=0.005% reflectivity loss, which corresponds to a carbonthickness of about 0.01 nm (C mono-layer is about 0.1-0.2 nm). Thus, aRu detector surface 11 may provide a very sensitive detector concerning,for example, carbon contamination.

Where the detector surface 11 substantially comprises Ruthenium (Ru),small quantities of contamination growing on optics, for example an EUVmirror, may be detected, before the contamination becomes a problem forthe optics. Generally, a mono-layer or smaller of contamination may bedetected.

For example, in an EUV lithography environment, mirrors becomecontaminated due to several reasons, such as Sn debris from an EUVsource, carbon growth from EUV illumination and outgassing of EUVresist. Since EUV optics are highly sensitive to contamination (1 nm ofcontamination may already be unacceptable), it is important to be ableto monitor the growth rate of contamination. As follows from the above,a solution is to measure secondary emission (SE) from, for example, a Rudetector surface 11.

In an embodiment, there is provided a method to clean a surface of anoptical component (for example to remove a layer of contaminants),comprising subjecting the surface to a cleaning treatment, andmonitoring the surface by irradiating the surface such that secondaryelectrons are emitted by the surface and detecting a resulting secondaryemission current or voltage. Via the monitoring, for example, it may bedetermined whether the cleaning works and when the cleaning is complete.Thus, a relatively simple and accurate method is provided to monitor thecleaning process of an optical component, wherein the optical componentprovides the above-mentioned secondary electron emission surface 11.

According to another embodiment, part of which is schematically shown inFIG. 6, there is provided a lithographic apparatus configured to projecta patterned beam of radiation onto a target portion of a substrate W,the apparatus comprising a contaminant sensitive radiation dose detector10, 10′ sensitive to a contaminant that is likely to contaminate thedetector during operation of the apparatus, the detector 10, 10′comprising a secondary electron emission surface configured to receive aradiation flux and which may also receive the contaminant, andconfigured to emit secondary electrons due to the receipt of theradiation flux, wherein a meter (not depicted in FIG. 6) is connected tothe detector surface to detect a current or voltage resulting from thesecondary electron emission. In this case, the detector is positioned toreceive a contaminant emanating from the substrate W during operation,as is depicted in FIG. 6.

In an embodiment, the apparatus may comprise a projection system PS toproject the radiation beam onto the substrate W, and a gas shower 40 toprovide a gas curtain between the substrate W and the projection systemPS. The gas shower 40 as such is known to the skilled person, and may beconfigured in various ways. Particularly, one or more gas showers may beconfigured to shield the downstream part of the projection system fromthe substrate zone, using one or more suitable gas flows. For examplethe gas shower(s) 40 may be configured to substantially preventcontamination originating from the substrate W to reach the projectionsystem PS.

In an embodiment, the above-described radiation dose detector 10, 10′may be located in at least one of the following locations: between thesubstrate and projection system, in the gas curtain, between the gascurtain and the substrate, or between the gas curtain and the projectionsystem.

Two examples of the positions of a radiation dose detector 10, 10′ areindicated in the FIG. 6, a first detector 10 being located in a regionbetween the substrate table WT and gas shower(s) 40, and a seconddetector 10′ being located in a region between the projection system PSand gas shower(s) 40.

For example, the detector 10 may be used to be able to monitor theamount of contamination induced by resist on the substrate W. In thisembodiment, the amount of contamination due to resist outgassing (forexample hydrocarbons, Si-hydrocarbons, F-hydrocarbons) from thesubstrate W may be monitored. For this purpose, as follows from FIG. 6,the detector 10 may be placed in close proximity to the substrate W.

Sometimes, one may accidentally use a bad type of resist that evaporatestoo much material, resulting in a rapid growth of contamination on theEUV optics. It is advantageous if a sensor would be able to detect thisin an early stage, such that an alarm may be given to stop the use of aparticular substrate or a particular type of resist. In the EUV tool, agas curtain may be used directly above the substrate, in order toprevent particles originating from the resist from reaching the EUVoptics. However, in some cases this gas curtain may not be efficientenough, especially if a bad resist is used.

In the case the detector 10 is placed between the substrate W and thegas curtain 40, the detector is desirably closest to the contaminationsource (i.e. substrate W) and will thus be the most sensitive.Typically, the suppression of contaminants by the gas curtain is 4orders of magnitude, thus the sensitivity of the detector with respectto contamination process on the projection optics PS behind the gascurtain may be enormous.

In another example, a detector 10′ be placed directly behind the gascurtain. An advantage of this location is that the detector is partiallyprotected from contamination by the gas curtain, and the detector islocated in same area where the EUV optics are.

Also, for example, the detector 10 may be spaced-apart from a substratetable WT or gas shower 40. Alternatively, for the detector 10 may beintegrated with a substrate table WT or gas shower 40. When the detector10 is placed somewhere within the gas curtain, it may be used in orderto tune the accuracy of the detector to a desired value.

As is shown in FIG. 6, illumination/irradiation of the detector 10, 10′may be stray radiation r1 of the EUV beam used for operation of thetool, or radiation r2 emitted by an alternative low power source 50(schematically depicted in FIG. 6), or another wavelength small enoughto create photo-electrons. Alternatively or additionally, the detector10, 10′ may be arranged to detect at least part of the EUV beamdirectly.

In an embodiment, there may be provided an above-described radiationdose detector 10A, 10B, 10F, 80 (see FIGS. 1 and 7) to monitor debrisemanating from a radiation source SO of or associated with theapparatus. As follows from the above, the detector may be provided by asecondary electron emission surface 11, configured to receive aradiation flux and optionally contamination, and to emit secondaryelectrons due to the receipt of the radiation flux, wherein a meter isconnected to the detector 10A, 10B, 10F, 80 to detect a current orvoltage resulting from secondary electron emission from the electronemission surface 11.

For example, in addition to radiation (for example, EUV radiation), aradiation source SO typically generates debris. For example, an Sn-basedEUV source will generate Sn particles, which travel further downstreaminto the EUV system. In order to block these particles, a debrismitigation system may be employed, using for example a foil trap FT anda buffer gas. At some stage, these components may also be cleaned usinga Sn cleaning method such as halogen cleaning or hydrogen cleaning.

Even though most source related contaminant particles are blocked by thedebris mitigation system, some source debris may still travel furtherinto the apparatus. Thus, it is important to monitor the amount ofcontamination near the radiation source SO. For example, in order tomonitor Sn contamination, a detector 10A may be placed, for example,near a (first) intermediate focus 75 (either behind, or directly before)downstream with respect to the source SO.

In an embodiment, an aperture 80 may be present at the intermediatefocus 75 downstream of the source SO, for example to block radiationthat cannot be projected due to etendue limits. This aperture 80 will bepartially illuminated by the radiation (for example EUV), and may thusbe used to measure secondary emission and thus contamination growth. Forexample, as is depicted by FIG. 7, at least part of a surface of anaperture component 80 of the apparatus may be provided with theabove-described secondary electron emission surface 11, wherein a meter13 may be arranged to measure a secondary electron emission current orvoltage.

Alternatively or additionally, for example, a separate secondaryelectron emission surface 11 may be located directly in front oralternatively behind the intermediate focus 75. As an example, in FIG.7, a detector 10B is located in front of the aperture 80 as well asanother or alternative detector 10B′ behind the aperture 80.

Additionally or alternatively, a detector 10F (or at least a detectorsurface 11) may be placed somewhere within a radiation collector K, orbe part of the collector K, as is depicted with broken lines. Similarly,a detector 10A′ (or at least a detector surface 11) may be placedsomewhere within a foil trap FT, or be part thereof. Similarly, adetector 10A″ may be located between a foil trap FT and a source SO,directly looking at the source SO, and/or a detector 10A may be betweena foil trap FT and a collector K.

In an embodiment, contamination growth may be monitored near theprojection optics PS and illumination optics IL of the lithographysystem. Typically, in that case, contamination may be hydrocarbons, butalso Sn debris or oxidation may be important here (due to for example atemporary loss of the vacuum specifications, or a leak in the vacuumsystem). In order to monitor this contamination growth, one or more ofthe above-mentioned detectors 10C, 10C′, 10D, 10D′ is/are placed withinor near the projection optics PS and/or the illumination optics IL (seeFigure I), for example within part of the radiation beam.

The embodiment of FIGS. 8 and 9 may provide a simple way to monitor anamount of ion-induced sputter etching that may occur in, for example, anEUV environment. Herein, as follows from the above, a current or voltageinduced by secondary electron emission from a multi-layer structure, ismonitored, for example by the data processor 115. By monitoring changesin the current or voltage, the data processor 115 may distinguishbetween different layers 110(M1), 110(M2) that are being etched away,and thus may tell how much etching is occurring.

The embodiment of FIG. 8 may be combined with one or more of theabove-described embodiments, if desired. An embodiment of FIG. 8comprises a lithographic apparatus configured to project a patternedbeam of radiation onto a target portion of a substrate, the apparatuscomprising an etch detector 110 comprising a detector body having asecondary electron emission surface 111, the surface configured toreceive a radiation flux and to emit secondary electrons due to thereceipt of the radiation flux, wherein a meter 13 is connected to thedetector body 110 to detect a current or voltage resulting from thesecondary electron emission, wherein the composition of the detectorbody varies 110, measured in a direction perpendicular from the electronemission surface 111. In an embodiment, a data processor 115 is providedto detect etching of the detector 110 from the current or voltagemeasured by the meter 13. For example, the detector 110, meter 13 anddata processor 115 may be configured like the above-described examplesthereof.

In the present embodiment, since the composition of the detector bodychanges 110, measured in a direction perpendicular from the surface 111,etching of the surface 111 may be detected in a simple manner,real-time, for example by the data processor 115.

For example, the detector body 110 may comprise: different layersconsisting of different materials; at least one (compositionally) gradedlayer having a gradually changing composition; different graded layers;at least one layer having a thickness of about 1 nm or less; or anycombination of the above.

More particularly, the embodiment shown in FIG. 8 may comprise amulti-layer detector structure 110 and a meter 13 configured tomeasuring current or voltage induced by secondary electron emission fromthe detector surface 111. For example, as is shown in FIG. 9, thedetector multi-layer structure 110 may comprise a substrate 110(S)comprising alternating layers 110(M1), 110(M2) of different materials(M1, M2), which materials (M1, M2) give a different secondary electronemission current when illuminated with the radiation R (see FIG. 8). Forexample, the multi-layer stack 110(M1), 110(M2) may comprise only firstlayers 110(M1) consisting of a first material, and second layers 110(M2)consisting of a second material that differs from the first material. Itis also possible to apply more than 2 different layers. Each of thementioned layers may be provided by one or more materials (for example amixture). Also, as an example, the detector 110 may be provided with oneor more intermediate strengthening layers (located between first andsecond layers 110(M1), 110(M2)), configured to reduce thermal expansionstress between the layers of different materials. As an example, adetector layer 110(M1), 110(M2) may comprise or consist of: a metal,carbon, Ruthenium, Molybdenum (Mo), a semiconductor, silicon, an oxideand/or other suitable materials, or any combination of the foregoing, aswill be appreciated by the skilled person. In an example, the firstlayers may comprise silicon and the second layers comprise Molybdenum.

In an embodiment, there may be provided compositionally graded layers inthe detector structure 110. Here, the concentration of a certain elementmay continuously change as a function of depth, particularly such thatvariation of secondary electron emission will occur in the case of theabove-described etching of the detector structure 110 (while the sameradiation flux is applied to the detector structure 110 to induce suchelectron emission). An advantage of such an embodiment is that it allowsthe etching rate to be measured more continuously.

In an embodiment, one or more optical components of the apparatus may benegatively affected by undesired etching processes, for example due toEUV induced background plasma in the system and ions emanating from theradiation source SO. As an example, ultimately, the lifetime of aradiation collector K may be limited by the etching/sputtering of ions.Thus, it is important to be able to monitor the amount of etching thatoccurs near the optics in the lithography tool.

For example, the etch detector 110 may be placed near one or moreoptical elements in the apparatus that may suffer from etching, forexample in one or more of the similar positions of the detectors 10 asshown in FIGS. 1-7. For example, the sensor 110 may be placed at anabove-described intermediate focus in order to monitor the expectedamount of etching of a multi-layer mirror downstream of the intermediatefocus. In another example, the sensor may be placed near the source SO,or within the collector K, in order to monitor the etch rate of thecollector.

In an embodiment, there may be provided a multi-layer structure orstack, for example a multi-layer mirror element of a radiationcollector, comprising different layers of different materials (forexample a stack of silicon and Molybdenum layers), such that thedifferent layers will provide different amounts of secondary electronemission upon exposure. In that case, following the above-describedmethod, the amount of etching of the multi-layer collector element maybe monitored in a simple manner, by detecting variation in the electronemission current or voltage due to subsequent etching away of subsequentstack layers. Particularly, it may be monitored how many of the mirrorlayers have already been etched away.

In the above-described embodiments, illumination/irradiation of thedetector (or monitor) 10 may be direct irradiation from radiationgenerated by the radiation source SO of the apparatus. However, thismeans that the detector 10 will block some of the radiation beam, whichmay sometimes not be possible/acceptable. In an embodiment, the detector(or monitor) 10 is placed near the radiation beam (but not directlywithin the beam). Radiation that is scattered within the system may thenilluminate the detector surface 11, generating secondary emission. Ashas been mentioned, an alternative power source may also be provided toirradiate the surface 11 of the detector 10.

In an embodiment, a vacuum may be characterized and/or controlled. Forexample, an EUV optical system (see FIG. 1) may be operated underoptimized vacuum conditions that are currently controlled mainly byresidual gas analyzer measurements. The vacuum conditions together withthe illumination itself may be responsible for contamination of anoptical surface, for example of a mirror surface of the above-describedoptics. The process of contamination of the optical surface may consistof both adsorbing of volatile and non-volatile organic and inorganiccompounds and the interaction of these adsorbents at the optical surfacewith the incoming radiation.

In an embodiment, advantageously, a dedicated method is proposed,wherein contaminant status of an optical surface (of an above-describedoptical component) is checked to draw detailed conclusions on the vacuumcondition under which illumination has been performed. For example, thismethod may be indirect, but sensitive for vacuum qualification (at leastmuch more sensitive than a residual gas analyzing method as describedabove), as well as direct for checking the contamination/degradationstatus of the observed optic, and with this the optics mean time betweencleaning and of lifetime maybe determined. In this embodiment, one ormore optical surfaces may be used in the vacuum environment to bechecked. The one or more optical surfaces may be analyzed in situ or exsitu, for example after a certain monitoring period and desirably whilebeing irradiated, to characterize the vacuum. The analysis may becarried out in various different ways, for example by outgassing theoptic and detecting contaminants emanating from the optic and resultingfrom that outgassing. Another method includes determining reflectivity,in the case the optic is a mirror, or secondary electron (e.g., current)measurements.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus and/or collector describedherein may have other applications, such as the manufacture ofintegrated optical systems, guidance and detection patterns for magneticdomain memories, flat-panel displays, liquid-crystal displays (LCDs),thin-film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“wafer” or “die” herein may be considered as synonymous with the moregeneral terms “substrate” or “target portion”, respectively. Thesubstrate referred to herein may be processed, before or after exposure,in for example a track (a tool that typically applies a layer of resistto a substrate and develops the exposed resist), a metrology tool and/oran inspection tool. Where applicable, the disclosure herein may beapplied to such and other substrate processing tools. Further, thesubstrate may be processed more than once, for example in order tocreate a multi-layer Ie, so that the term substrate used herein may alsorefer to a substrate that already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to anyone orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A method comprising: arranging a detector and an optical component such that a surface of the detector and a surface of the optical component receive similar levels of contamination, wherein the surface of the detector is separate from the surface of the optical component; subjecting the surface of the detector and the surface of the optical component to a cleaning treatment; irradiating the surface of the detector such that secondary electrons are emitted by the surface of the detector; and detecting at least one of a resulting secondary emission voltage and current.
 2. The method of claim 1, wherein the cleaning treatment comprises subjecting the surface of the detector and the surface of the optical component to hydrogen radicals.
 3. The method of claim 2, wherein the cleaning treatment further comprises: providing hydrogen gas in at least part of the hydrogen radical supply system; producing hydrogen radicals from the hydrogen gas; and subjecting the surface of the detector and the surface of the optical component to at least part of the hydrogen radicals.
 4. The method of claim 3, wherein the producing the hydrogen radicals from the hydrogen gas comprises using at least one of a filament, a plasma, radiation, and a catalyst.
 5. The method of claim 1, wherein the cleaning treatment removes at least part of a deposition from at least one of the detector surface and the optical component surface, and wherein the deposition comprises at least one of B, C, Si, Ge, and Sn.
 6. The method of claim 1, further comprising: comparing a voltage or current corresponding to the resulting secondary emission to a stored secondary emission voltage or current.
 7. The method of claim 1, further comprising: storing the resulting secondary emission voltage or current. 